User`s manual

Postion Feedback Devices
GFK-1742A Appendix C Position Feedback Devices C-3
C
Set Position Command - Absolute Encoder Mode
The
Set Position
%AQ command functions the same way as in incremental encoder mode. At the
completion of the
Set Position
operation,
Actual Position
is set to the
Set Position
value. The
DSM314 internally calculates the encoder Absolute Feedback Offset needed to produce the
commanded
Set Position
value. This Absolute Feedback Offset is immediately saved in the
DSM314 non-volatile (capacitor backup) memory.
If a Set Position AQ command is received before the encoder has been referenced, Error Code
53(hex) “Attempt to initialize position before digital encoder passes reference point” will be
reported. This error code is only reported if the Feedback Mode is set to
Absolute
. Serial
Encoders configured for
Incremental
mode do not have this restriction.
Once an absolute position is established by a
Set Position
command, the DSM314 will
automatically initialize
Actual Position
after a power cycle and set the
Position Valid
%I bit.
Absolute Encoder Mode - DSM314 Power-Up
The battery pack attached to the servo subsystem will maintain power to the encoder counter logic.
Once the encoder has referenced through first time start up, the actual position is automatically
maintained by the encoder, even if the axis is moved during servo power loss. The encoder will
monitor the status of the battery pack, and report loss of battery power or low battery power to the
DSM314.
The DSM314 will complete a power-on diagnostic, and when configured for absolute encoder
mode, interrogate the referenced status of the Digital serial encoder. A valid referenced status
from the encoder will signal the DSM314 to read the encoder absolute position. The DSM314
will report the
Actual Position
%AI status as the sum of the encoder position and the Absolute
Feedback Offset established by the initial Find Home cycle or
Set Position
%AQ command.
Incremental Quadrature Encoder
Incremental Quadrature Encoders provide three output signals to the DSM314: Channel A,
Channel B, and Marker. The Channel A and Channel B signals transition as the encoder turns,
allowing the DSM314 to count the number of signal transitions and calculate the latest encoder
position change and direction of rotation.
Incremental Quadrature Encoders are incremental feedback devices; they do not provide a
continuous indication of absolute shaft angle as the input shaft rotates. For this reason, the
DSM314’s
Actual Position
%AI status word must be initialized with a known physical position
before positioning control is allowed. This position alignment can be accomplished using the
Set
Position
%AQ Immediate command or the
%Q Find Home
cycle . The home cycle makes use of
the encoder marker channel, which is a once per revolution pulse produced at a known encoder
shaft angle. Successful completion of the %Q
Find Home
cycle or a
Set Position
%AQ command
causes the DSM314 to set the axis
Position Valid
%I bit.
Position Valid
must be set before motion
programs will be allowed to execute.
Position Valid
is only cleared by an encoder Quadrature
Error (Channel A and Channel B switching at the same time) or by turning on the
Find Home
and
Abort
%Q bits simultaneously.
Note
In Digital Mode, only incremental quadrature encoders are supported for the
Follower mode master axis.